Update README formatting and content

This orders the phase subdirectories by execution order, adds
subheadings, and fixs some markdown formatting issues.

Change-Id: I2d5526cbe4a902067fa9cad1456c35d13f81e183
typographic purity - single space after full-stop

Change-Id: I7edc239071eaab78c55c23f6886bdfd99724c1c6